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Purchasing Public IP Addresses and not being able to use all of them
When buying public IP addresses, why is it that you need to buy some that don't actually respond or get used? I am told they are used for broadcasting but not quite sure how or why? E.g. about to buy 4 but only 2 can be 'used' I am told....
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Answers
IP addresses are a mix of the addresses and the subnet mask. your provider may buy a block like
66.55.0.0 /16 (subnet mask 255.255.0.0)
then they'll resell smaller blocks like 66.55.1.0 /24 (255.255.255.0)
the subnet mask tells the network which bits are used to identify a specific host and which bits are used to identify a specific subnet. When a service provider splits up these addresses into smaller blocks they need to be able to tell what string of bits are a valid host address versus a valid subnet. So there is a need to identify a "network" address to which a whole range, or subnet, of addresses can be routed to.
One IP (typically the first) is used to identify the subnet, the other is a broadcast address. Traffic sent to this address is "broadcast" to all IP's in that subnet. This is in part a holdover but is also ust for things like ARP queries. If you put a device on a broadcast IP it interferes with normal operatio of services that require the ability to query the whole subnet.
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by: skca54Posted on 2007-04-16 at 03:06:10ID: 18916615
One IP address will be allocated permanently to your router so that the IP supplier knows to route the entire block to that connection. Then you will have two other's to use. However many IPs you buy you will always lose two addresses.